One area of interest for its ability to produce a net reduction in lifecycle carbon emissions comes in the form of alcohols produced by fermentation. Fermenting soluble sugars to produce ethanol or butanol is known in the art. While fermentation of soluble sugars may represent a way to energy self sufficiency for petroleum-challenged regions, the net lifecycle carbon dioxide emissions may actually exceed those of petroleum diesel and gasoline depending on the source of sugar and the method of its fermentation. For example, there is some debate as to whether ethanol produced from the fermentation of soluble sugars in corn grain consumes more carbon based energy than it produces. Not only is a great deal of fossil energy expended during the planting and harvesting of grain corn, but large amounts are required during the manufacture of ethanol—especially due to the water/alcohol separation and byproduct drying steps. Furthermore, carbon dioxide is a significant byproduct of fermentation itself. High soluble sugar content materials such as sugar beets and cane can increase net energy and carbon efficiency only to some degree.
One approach to achieving positive net energy production is to convert insoluble sugars such as cellulose from widely available lignocellulose material to soluble sugars that can be fermented. For example, the production of corn grain also yields a comparable amount of lignocellulosic material that is currently underutilized. The yield of grain ethanol from corn grain is about 29 wt %. The mass of corn stover to grain is roughly 1:1 and processes for recovering 20 wt % ethanol from stover have been commercialized. Converting the cellulose in stover to soluble sugar (a process known as saccharification or hydrolysis) consumes additional energy relative to that of simply tilling the stover back into the ground. However, the 70% increase in ethanol production compensates for the additional energy requirements causing the overall process to become respectably net energy productive.
The US Departments of Agriculture and Energy have estimated that the current availability of corn stover for use in ethanol production, without any change to current tillage or land use practices, to be about 75 million tons. If other lignocellulosic crop wastes are considered, the available cellulose from current agricultural practices is in excess of 190 million tons per year (Table 3). (U.S. Department of Agriculture and U.S. Department of Energy. BIOMASS AS FEEDSTOCK FOR A BIOENERGY AND BIOPRODUCTS INDUSTRY: THE TECHNICAL FEASIBILITY OF A BILLION-TON ANNUAL SUPPLY″. April, 2005.) If no-till practices are adopted and crop yields increased, the amount of biomass available for fuel production can be increased to approximately 500 million tons per year (Table 4). Further expansion of available biomass to nearly a billion tons per year is achievable by increased farming of perennials such as switch grass (Table 5).
Forestry offers another source of cellulose for the production of ethanol. The US Departments of Agriculture and Energy have estimated that the current availability of cellulose from forest resources stands at 142 million tons per year and is expandable to 368 million tons per year (Table 6).
Underutilized cellulose from agriculture and forestry represents a resource for the production of net energy positive ethanol. Assuming the 1 billion tons per year or so that USDA and DOE estimate to be within reach along with a 20 wt % yield leads to over 60 billion gallons of ethanol production per year with a net reduction in CO2 production. These resources can be realized only if the sugars locked in biomass in the form of insoluble cellulose can be separated from associated lignin and transformed into soluble sugar via saccharification/hydrolysis. Diverse technologies for accomplishing this separation exist at varying stages of investigation or commercialization. (“Costs Prohibit Cellulosics Use as Feedstock”. C&EN, Apr. 12, 1976, pg. 12.)
Despite the investigation of ethanol as an alternative to petroleum to reduce carbon emissions, there remains a need for economically-viable energy alternatives such as plant and animal-derived ester-based fuels for controlling carbon dioxide emissions during the production of energy.